The inactive Protein Kinase A (PKA) complex contains 2 regulatory subunits and 2 catalytic subunits. Binding of the regulatory subunits to the catalytic subunits maintains inactivity. In humans there are 3 different catalytic subunits and 4 different regulatory subunits. The particular subunits present in the beta cells of the pancreas are unknown. In beta cells PKA is associated with AKAP79 and IQGAP1, which are believed to tether PKA to the inner surface of the plasma membrane. Activation by cAMP occurs when each regulatory subunit binds 2 molecules of cAMP, causing dissociation of the catalytic subunits. The active catalytic subunits are thereby released to phosphorylate their target proteins.Prolonged exposure to increased cAMP levels results in translocation of the active catalytic subunits to the nucleus, where they regulate the PDX-1 and CREB transcription factors and cause increased transcription of the insulin gene.